Mark scheme. 1 Forces and motion 1. Practical work. 1 Forces and motion 1

Transcription

1 Forces and motion Mark scheme Forces and motion Practical work States a similarity, e.g. both experiments measure the fastest speed States a difference, e.g. one experiment takes readings using a data logger States which method is more accurate, e.g. using a data logger Explains why one method is more accurate, e.g. using a data logger reduces human error such as reaction time List two control variables, e.g. the mass of each seed; the height it is dropped; how the seed is released; where/how measurements are taken Repeat readings for each seed to spot anomalous results/reduce random error Work with a partner, e.g. to measure time or distance more accurately Choose seeds with very different areas to give a wide range of results (Any other valid response with an explanation) Total 5 3 The reading from the newton meter is wrong because it is not zeroed properly/they do not look at it from the same position The force is different each time because the door is not in the same position/the newton meter is not pulled in the same direction each time (Any other valid reason with an explanation) 4 a) Mass of ball Time to fall a fixed distance b) The tube is longer than the measuring cylinder so the distance can be longer so timing becomes more accurate c) More than one variable changes/variables are not controlled. For example, the ball s shape or surface may change as well as its mass/there is friction between the tube and ball Total 7 Hodder & Stoughton 03

2 Mark scheme Graphs Question Answers and guidance Mark a).5 m/s (answers between.4 and.6 m/s are acceptable) b) A A has a larger change of speed in the same time/the gradient of the graph is equal to the acceleration and A has the steeper gradient, so larger acceleration c) d = v t = 0 6 = 60 m Or use the area under the graph d) 5 B velocity/m/s 0 5 A time/seconds a = (v - u)/t = (5-5)/4 =.5 m/s ( mark for the correct unit) Total 7, a) 45 km (± km) b) 9.6 h (or 9 h 36 min) (answers between 9.5 h and 9.7 h gain the mark) c) h The graph is horizontal between C (5 h) and D (6 h). The distance does not change, which shows the man is not moving. The time difference is h d) Between D and E Here the gradient of the graph is the shallowest (The gradient gives the speed.) e) average speed = distance/time = 4/ = 7 km/h (Make sure you write the equation, put in the numbers and include the unit with the answer. You lose mark if the unit is missing) f) Both speed and velocity are measured in m/s but velocity also has a direction Total Hodder & Stoughton 03

3 Forces and motion 3 a) acceleration = change of speed/time = 0/4 =.5 m/s b) acceleration = change of speed/time = (30 0)/0 =.0 m/s c) As the speed increases, the drag increases. So the unbalanced forward force decreases, which reduces the acceleration. d) The distance travelled can be calculated from the area under the graph up to 0 seconds This is closest to 350 m. (It is difficult to calculate this accurately, because the graph curves) e) speed/m/s first plane more massive plane f) i) time/s Two marks for drawing a curve below the first one. A larger mass causes the acceleration to be less 9000 N; the vertical speed is zero, so the vertical forces balance 600 N; the horizontal speed is constant, so the horizontal forces balance g) m = W/g = 9000/0 = 900 kg Total 6 Calculations a) W = mg = g g = / =.6 N/kg b) = N c) F = ma = a a = 5 000/ = 0.5 m/s d) a = change of speed/time 0.5 = change of speed/60 change of speed = = 30 m/s Total 0, Hodder & Stoughton 03 3

4 Mark scheme a) acceleration = change of velocity/time = 8/.7 = 6.4 m/s b) i) F = m a = = 97 N ( 000 N) The drag force increases with speed. At 50 m/s the resultant force is less, so the acceleration is less c) The total mass is less, so the same force produces a larger acceleration d) i) acceleration = change of speed/time = (84 3)/.3 = 40 m/s (Remember to calculate the change in speed.) F = m a = = 300 N (This is four times the driver s weight) e) Three for mark each from: Worn tyres will reduce the grip this will increase braking distance. Worn brakes will reduce the braking force this will increase the braking distance. The car is less massive as it has used fuel this will reduce braking distance. Oil or rubber marks on the track might reduce grip this will increase the braking distance. Rain on the track would reduce the grip this would increase the braking distance f) Two safety features, with mark for naming the feature and mark for the explanation, up to a maximum of 4 marks: Crumple zones these increase the time of a collision and reduce the force of such an impact on the driver A rigid cage for the driver this protects the driver in a collision. He is less likely to get hit by something sharp A safety belt for the driver this enables him to benefit from the time it takes the crumple zones to collapse A neck brace for the driver this helps to protect him from large forces on his neck (whiplash forces); these could act forwards, backwards or sideways as he accelerates, decelerates or changes direction A low centre of gravity in the car this makes it more stable A wide wheel base this makes the car more stable Wide tyres these improve the grip on the road A crash helmet the hard outside of the helmet protects the head from small objects thrown up from the track. The inside of the helmet is soft. This reduces the force from a blow to the head, by absorbing the energy over a longer distance (or any other feature, with a correct explanation) Total Hodder & Stoughton 03

5 Forces and motion 3 a) momentum = m v = 50 6 = 300 kg m/s ( mark for the correct unit) b) force = change of momentum/time = 50 6/0. = 500 N (Remember to write the equation, put in the numbers, and then give the answer with the unit there is mark for the correct unit) c) Paul has momentum when he lands When he bends his legs, the momentum changes more slowly This makes the force less, so he is less likely to hurt himself You can also quote this equation to support your answer: force = change of momentum/time This question can also be answered using energy: Paul has kinetic energy when he lands. When he bends his legs, energy is transferred into elastic energy in his leg muscles as they stretch Bending the legs makes the distance longer, and the force on him less. So he is less likely to get hurt Total 9 Application of knowledge a) You must state whether you agree or disagree, but this is not awarded a mark. Identifies a trend from graph, e.g. stopping distance is shorter at all tread depths Uses data from the graph, e.g. stopping distance on asphalt is the same for 7 mm and 4 mm tread depth States an opinion, e.g. road surface is a significant factor b) States a suitable tread depth, e.g. 4 mm Gives a valid reason, e.g. tread depths deeper than 4 mm don t reduce stopping distance on asphalt Total 5,, Marks are not for the choice, but for the explanation; mark per bullet point Either answer is acceptable, but answers must be consistent with the choice made EITHER install speed cameras: Drivers are more likely to slow down and avoid an accident Slower speeds reduce the force felt by a pedestrian (momentum changes more slowly) Slower speeds cause less injury/death Use example from graph, e.g. death rate falls from 00% at 55 mph to 50% at 45 mph OR redesign car bonnet: Drivers can ignore speed cameras but the car shape can t change Use example from graph, e.g. even at 30 mph, the risk of serious injury is 40% Injuries are reduced at all speeds if the bonnet is modified A smooth bonnet/bonnet that deforms easily reduces the force felt 4 Hodder & Stoughton 03 5

6 Mark scheme 3 Choice of feature plus a reason, e.g. crumple zone deforms in crash and absorbs energy OR seat belts hold person in place and stretch slightly OR air bags inflate and cushion the person Plus any three for mark each from: momentum = mass velocity rate of change of momentum = force felt the safety features increase the duration of impact this reduces the force on passengers a lower force reduces injury 3 4 Theory, e.g. stopping distance = thinking distance + braking distance Theory, e.g. reducing the speed limit reduces thinking distance and braking distance; treating roads reduces braking distance only Data from the graph is used to compare treated/untreated surfaces, e.g. braking distance on treated roads reduces by m compared with ice or 9 m compared with snow Concludes that reducing speed limit would have more effect on stopping distance 6 Hodder & Stoughton 03

7 Forces and motion Forces and motion Practical work Any four points for mark each: Identifies parameters, e.g. measure the original length of the spring States the dependent variable, e.g. the new length of the spring/ extension Describes use of equipment, e.g. use a ruler to measure length Describes how to change the independent variable, e.g. add weights one at a time Describes one example of data handling e.g. calculate the extension a) Move the loop of string along the ruler b) Choose any three ( mark each) from: Use a clamp stand to hold the force meters so they do not move/ make sure the position of force meters does not change between readings/keep the height of force meters the same Check the position of the loop of string on the ruler at eye level to avoid misreading it/errors from parallax Use thin string so its position can be seen precisely Use a metre ruler with mm divisions for precise readings c) Measure distance from one end of the ruler and the reading on the force meter at the same end Plot a graph of distance against force Total a) moment = force perpendicular distance from the pivot b) When the beam is balanced, measure the perpendicular distance from the pivot to the N weight Measure the perpendicular distance from the pivot to the wood The moments are equal either side of the pivot when it balances Weight of wood = ( N distance from pivot to weight)/(distance from pivot to wood) c) Hard to estimate the exact position of the wood/weight Hard to balance the ruler exactly Total 7 Hodder & Stoughton 03 7

8 Mark scheme Graphs a) radius of orbit/000 km b) time/days Axes labelled Good choice of scale (the x- and y-axes must use at least half the page) Points plotted accurately Line of best fit drawn Line drawn from 3.5 days to meet the line of best fit. A line drawn from that meeting point to the y-axis. Correct reading km ( km to km accepted) Total 7 a) load/n.0 spring rubber band extension/cm Axes and graphs labelled Good choice of scale Points plotted accurately Two lines of best fit drawn (lose mark for the wrong axes) 8 Hodder & Stoughton 03

9 Forces and motion b) The extension is a continuous variable as it can take any value (Here the load is a discrete variable as it has been changed in discrete or separate steps of 0.5 N.) The extension is the dependent variable as it depends on the load which is the independent variable c) point identified by ringing it or labelling it.5 N.0 cm. (.5 N.9 cm also accepted) d) Range of acceptable values 3.8 to 3.9 N (Remember to draw lines on your graph to help you get the correct answer) e) Hooke s law states that the extension is proportional to the load The spring obeys Hooke s law up to a load of 3.0 N f) Elastic means that the spring returns to its original length when the load is removed Plastic means that the spring has been permanently deformed and does not return to its original length when the load is removed g) Debbie has improved the precision of the experiment by using smaller loads such as 0. N, 0. N, etc. When she has plotted the graph she can see that the extension is proportional to the load up to a small limit such as 0.5 N Total 9 3 a) i) i b) The speed falls steadily from 7600 m/s at a height of 500 km, to a speed of 600 m/s at a height of 4000 km The atmosphere stops satellites from orbiting m/s The pull of gravity keeps the satellite in orbit. Gravity gets less at larger distances Total 6 Calculations a) gravity b) i) c) i) 8000 km (the radius of the Earth plus the height of the orbit) v = πr/t = π /0 60 = 698 m/s An ellipse or elliptical orbit Three relevant points for mark each: At a large distance the satellite travels slowly so it has low kinetic energy, and high gravitational potential energy When it falls in towards the Earth, its kinetic energy increases and potential energy decreases The total energy stays constant; so the satellite stays in orbit permanently (This is hard to answer in full, but you can still earn 3 marks for some good relevant comments, even if your answer is not quite complete) Total 9 Hodder & Stoughton 03 9

10 Mark scheme a) W = m g = 40.8 = 5 N b) When he jumps he has the same kinetic energy on the Earth and Io Io has a weaker gravitational pull It takes him further to come to a rest and fall back on Io OR He has to go higher to transfer the kinetic energy into gravitational potential energy OR He does the same work when he jumps, so goes higher in a weaker gravitational field (You can get up to 3 marks for useful ideas, so keep writing even if you have difficulty understanding everything about the question.) c) radius of orbit = m time period of orbit = s = 5 00 s v = π r/t = π /5 00 = 7536 m/s (Make sure you turn the radius of orbit into metres, and the time into seconds before you start the calculation) Total 9 OR OR up to 3 3 a) moment = force perpendicular distance = = N m b) the sum of the anticlockwise moments = the sum of the clockwise moments 450 N 3 m = W.8 m W = 750 N Total 6 4 a) She can use a ruler to measure the height of the pivot above the bench top Then she needs to make sure that the middle of the other end of the ruler is the same height above the bench b) i) c) i) moment = force perpendicular distance (remember the word perpendicular) moment = 4 N 50 cm = 00 N cm the moment from the force meter must be 00 N cm to balance the weight 00 = F 0 F = 0 N The weight of the ruler also exerts a force The force meter must support this as well as the 4 N weight d) For the pole to be balanced the moment (about the vegetables) from A s force, must be balanced by the moment from B s force. The distance from A is larger than the distance from B. So the force from A is smaller than the force from B (This is a difficult question. You can still earn 3 marks for three good points even if the answer is incomplete), 0 Hodder & Stoughton 03